ldapsearch display certificate

hi
I got the problem to only display the usercertificate
using openldap ldapsearch tool.
I didn't get problem to display all attribute (
include the usercertificate) in this entries. But if I
specify the attribute "usercertificate" only , it
return nothing.
Is it the command line wrong?
ldapsearch -LLL -h localhost -p 1717 -b "cn=smith,
o=xxx, c=CA" -s subtree -D 'cn=manager,o=xxx,c=ca' -w
'password' objectclass='*' userCertificate
Thanks for help
Florence
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